This is an application for a Mentored Clinical Scientist Research Career Development Award (K08) for Michael LaFemina, MD, an Instructor in the Department of Medicine, Division of Pulmonary and Critical Care, at the University of California, San Francisco. Dr. LaFemina is establishing himself as a young investigator in the field of alveolar epithelial biology relating to acute lung injury. Specifically, this application s focused on the role of alveolar epithelial barrier disruption in the pathogenesis of acute lung injury1. Despite the requirement to strictly control barrier function in the alveolar epithelium2, ittle is known about the nature of tight junctions in the lung. Claudins are a family of transmembrane proteins that are both necessary and sufficient for tight junction formation3. Claudin-18, the only lung-specific tight junction protein4, is the most highly expressed claudin in type 1 pneumocytes5, suggesting a lung-specific function. Preliminary data indicate that (1) mice deficient in claudin-18 have increased alveolar permeability to albumin and the specific knockdown of claudin-18 in primary alveolar epithelial cells induces hyperpermeability; (2) claudin-18 null mice develop lung injury in the postnatal period, progressive fibrosis, and transforming growth factor-? (TGF?) activation; and (3) inflammation induces a specific loss of claudin-18 associated with increased permeability. Based on this preliminary data, the overall objective of this application is to elucidate the function of claudin-18 and to determine the role f this tight junction protein in the pathogenesis of acute lung injury. Aim 1 will test the hypothesi that claudin-18 is a unique paracellular barrier to macromolecules though studies of lung fluid balance in a novel claudin-18 deficient mouse and barrier function in primary alveolar epithelial cells deficient in claudin-18. Aim 2 will determine the mechanism of lung injury in the setting of claudin-18 deficiency and will investigate the link between barrier dysfunction, lung injury, and TGF-? activation. Aim 3 will determine the role of claudin-18 in clinically relevant models of acute lung injury through studies in ex vivo perfused human lung preparations. Dr. LaFemina's career development during this award will be organized around these aims and guided by formal mentorship and coursework. Dr. LaFemina has assembled a multidisciplinary team of UCSF senior investigators to mentor him: Dean Sheppard, MD, Professor of Medicine and an expert in integrin mediated activation of TGF-?; James Frank, MD, Associate Professor of Medicine and an expert in basic mechanisms of acute lung injury and tight junction biology; and Michael Matthay, MD, Professor of Medicine and Anesthesia and expert investigator in the basic and clinical science of acute lung injury. Dr. LaFemina's research proposal focusing on the novel protein claudin-18 coupled with his structured training plan will allow him to accomplish his long term goal career goal of becoming an independently funded physician scientist improving patient health through the study of basic mechanisms of alveolar epithelial biology. Disruption of the barrier between air and blood in the lung can lead to respiratory failure and death. Tight junctions are protein complexes that regulate this barrier. This study will investigate the role of the tight junction protein claudin-18 in lung injury and will determine whether this protein is a potential therapeutic target.